RESEARCH ARTICLE Copyright © 2012 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 12, 1–6, 2012 Spinodal Decomposition of Mono- to Few-Layer Graphene on Ni Substrates at Low Temperature Chan-Jung Hsu 1 , Pramoda K. Nayak 1 , Sheng-Chang Wang 2 , James C. Sung 3 , Chiang-Lun Wang 4 , Chung-Lin Wu 4 , and Jow-Lay Huang 1 5 6 ∗ 1 Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan (R.O.C.) 2 Department of Mechanical Engineering, Southern Taiwan University, Tainan County 710, Taiwan (R.O.C.) 3 KINIK Company, 64 Chung-San Road, Ying-Kuo, Taipei Hsien 239, Taiwan (R.O.C.) 4 Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan (R.O.C.) 5 Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan (R.O.C.) 6 Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 701, Taiwan (R.O.C.) Mono to few-layer graphene were prepared on pre-annealed polycrystalline nickel substrates by chemical vapor deposition at a relatively low temperature of 800  C using fast cooling rate. It was observed that the reduced solubility of Carbon in Ni at low temperature and an optimum gas mixing ratio (CH 4 :H 2 = 60/80 (sccm)) can be used to synthesize mano-layer graphene that covers about 100 m 2 area. The number of graphene layers strongly depends upon the hydrogen and methane flow rates. An increase in the methane flow is found to increase the growth density of the single-layer graphene. The number of graphene layers was identified from micro-Raman spectra. The thinnest areas containing mono-layer graphene formed at small Ni grains surrounded by large Ni Grains can be explained in terms of Spinodal decomposition. Scanning tunneling microscopy observations of the graphene samples indicate that the graphene structure exhibits no defects, and extremely symmetry hexagon carbon at flat graphene surface is observed. Keywords: Graphene, Scanning Tunneling Microscopy, Spinodal Decomposition. 1. INTRODUCTION Graphene, a single layer of carbon atoms bonded into two-dimensional (2D) hexagonal networks, has attracted considerable interest in solid state physics, material sci- ences, and nanoelectronics 1–3 since its isolation in 2004 by Novoselov et al. 4 as a free standing 2D crystal. The most unexpected and promising properties of graphene include thermodynamic stability, extremely high charge carrier mobility, and mechanical stiffness. For preparing next-generation graphene-based devices, large-scale syn- thesis of high-quality graphene is very important. Three main preparation methods for graphene include micromechanical exfoliation of highly ordered pyrolytic graphite, 5 6 ultrahigh vacuum graphization of silicon carbide, 7 8 and chemical vapor deposition (CVD) using transition metals as catalysts. 9–15 However, it is difficult to produce high-quality graphene with a large area for application as a practical electronic device material using ∗ Author to whom correspondence should be addressed. existing methods. On the other hand, clear understand- ing of substrate effect is also very important for poten- tial device application of graphene. Recently, syntheses of graphene sheets on different substrates have been stud- ied by some groups. 16 17 It is observed that the quality of micro mechanically cleaved mono-layer graphene is inde- pendent of the substrate used. Among the three main preparation methods mentioned above, CVD is attractive due to its low cost, scalability, and growth of single-layer graphene with a low defect density. The CVD approach relies on dissolving carbon into a metal substrate and then forcing it to precipitate out by cooling the metal. The thickness and crystalline order- ing of the precipitated carbon is controlled by the cool- ing rate and the concentration of the carbon dissolved in the metal. This concentration is in turn controlled by the type and concentration of the carbonaceous gas, and the thickness of the metal layer. After the chemical etching of the metal, the graphene membrane detaches and can be transferred onto another substrate. Direct CVD synthesis provides high-quality layers of graphene without intensive mechanical and chemical treatment. J. Nanosci. Nanotechnol. 2012, Vol. 12, No. xx 1533-4880/2012/12/001/006 doi:10.1166/jnn.2012.5799 1